Conventionally, in optical networks employing the WDM (Wavelength Division Multiplex) system, the optical transmission system having a plurality of transponders has been utilized in both ends of the WDM transmission section. The plurality of transponders correspond to a plurality of different wavelengths, respectively. The optical signal inputted to each transponder via an optical fiber from a router of the optical multiplex side is subjected to optical wavelength conversion at each transponder and then transmitted to the network side. The optical signal transmitted from each transponder is wavelength-multiplexed and then sent out to the WDM transmission section. Upon reaching the optical separation side via the amplification process by the optical amplifier on the WDM transmission section, the sent out optical signal is subjected to the optical wavelength separation. Upon receiving the optical signals on a wavelength basis obtained by the optical wavelength separation from the network side, the plurality of transponders disposed in the optical separation side output them to the corresponding routers in the client side. Thereby, the long distance transmission of the optical signal is implemented.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-522508    Patent Document 2: Japanese Laid-open Patent Publication No. 2010-028470    Patent Document 3: Japanese Laid-open Patent Publication No. 2005-341161    Patent Document 4: Japanese Laid-open Patent Publication No. 2012-099880
Due to the increase of the traffic amount in recent years, however, the transponder supporting 100 Gbps has been prevalent in addition to the related transponder supporting 10 Gbps. Therefore, the 10 Gbps transponder and the 100 Gbps transponder are mixed in one optical network. In the ITU-T (International Telecommunication Union-Telecommunication standardization sector), because multiple optical wavelengths are arranged in the wavelength grid with the interval of 50 GHz, the transponders for the different optical transmission speeds may be arranged neighboring to each other in the wavelength grid. In this case, due to the difference in the modulation system between the 10 Gbps and the 100 Gbps, the XPM (Cross Phase Modulation) that is the optical non-liner effect affects the 100 Gbps transponder of the phase modulation system from the 10 Gbps transponder of the direct modulation system. This causes the error rate of the 100 Gbps transponder to increase. This results in the reduced transmission quality in the optical transmission system.
The disclosed technique has been made taking the above into consideration and aims to provide an optical transmission apparatus and an optical transmission method that is able to quickly suppress the reduction in the transmission quality due to the addition of the neighboring wavelength.